This invention relates to optical pulse retiming and reshaping circuits, and finds particular, but not necessarily exclusive, application in the retiming of soliton pulses in long distance optical fibre data transmission systems in order to assist in overcoming the noise, inter-pulse interference, and the Gordon-Haus limitation introduced by the presence of concatenated optical amplifiers along the transmission path.
A paper by M. Nakazawa et al, entitled "10 Gbit/s Soliton Data Transmission over One Million Kilometers", Electronics Letters 4th July 1991 Vol. 27 No. 14, pages 1270-1272, discloses how a lithium niobate Mach Zehnder optical modulator can be used to assist in overcoming the Gordon-Haus limitation. In the experimental set-up described by those authors in the paper the solitons are recirculated in a 510 km long loop to synthesise the 106 km transmission path, and hence a single optical modulator used for pulse retiming and reshaping could obtain its drive signal via an electrical transmission path from a central clock, which was also used to generate the timing and shaping of the initially transmitted soliton pulse stream. In contrast, in a real long distance optical fibre transmission system it will normally be necessary to extract a local clock signal from the data stream at each pulse retiming and reshaping modulator location. The paper does not specify in detail, how this clock extraction can be effected, but merely states that it can be effected using a narrowband SAW filter. In the electrical regime clock extraction is conventionally achieved by tapping off some of the input data stream (electrical) pulses, amplifying the tapped-off signal, passing it through a high Q filter.
Such a clock extraction technique can be applied with minor modification as depicted in FIG. 1 for the extraction of an electrical clock signal from an optical data stream. FIG. 1 depicts an optical pulse retiming and reshaping circuit for inclusion in an optical data transmission system. Referring to FIG. 1, an optical modulator 10 is inserted into an optical data transmission line 11 for the transmission of return to zero (RZ) data pulses, typically soliton pulses. Just upstream of the modulator 10 is an optical tap 12 feeding a high speed photodiode 13. The output of this photodiode is fed via a high speed amplifier 14 to a high Q filter 15, typically a SAW filter, tuned to the frequency of the clock component of the data stream, to produce, at the output of the filter 15, an extracted clock signal. The clock signal is applied to a power amplifier 16 to amplify it sufficiently for use in driving the modulator 10. In FIG. 2 an illustrative short portion of a soliton pulse data stream which is applied to the input of the modulator is depicted by trace 20. The pulses of this portion are in need of retiming and reshaping. In particular the trace shows the optical amplitude as not returning to zero between the more closely spaced pair of soliton pulses. Trace 21 depicts the attenuation characteristic of the modulator 10 expressed as a function of time, this characteristic varying sinusoidally at the clock frequency between a substantially zero attenuation minimum and a high attenuation maximum. Trace 22 depicts the resulting retimed and reshaped portion of the soliton pulse data stream immediately after its passage through the modulator 10.
The optical pulse retiming and reshaping circuit of FIG. 1, though it uses high frequency electronics, is a much more simple device to implement than a full regenerator. A full regenerator has to handle broadband data as well as extract a clock signal, whereas in this circuit of FIG. 1 the only electrical signal is a narrow bandwidth clock signal. Narrow bandwidth external modulators are very much easier to make and drive, and the power requirements are much smaller than those of a full regenerator, being very little more than those of an amplifier alone.